Not Applicable
This invention relates to an apparatus and method for optimizing the performance of biological processes for wastewater treatment. In particular, the invention relates to optimization of the activated sludge process.
A variety of process combinations are used to treat wastewater before it is reused or discharged to the environment. Many of these combinations incorporate the activated sludge process. The activated sludge process was invented in the early 1900s by Ardern and Lockett (Ardem, E. and Lockett, W. T., xe2x80x9cExperiments on the oxidation of sewage without the aid of filters,xe2x80x9d Journal of the Society of Chemical Industries, 33, 523, 1914). Today, it is the most commonly used process for treating industrial and municipal wastewaters. The activated sludge process has many variations, including conventional activated sludge, extended aeration, pure oxygen activated sludge, step-feed, sludge reaeration, contact stabilization, and the solids contact step in the trickling filter/solids contact process, etc.
In the activated sludge process, wastewater is introduced to a biological reactor, termed an xe2x80x9caeration tankxe2x80x9d or xe2x80x9caeration basin,xe2x80x9d that contains a suspended culture of microorganisms, often termed xe2x80x9cbiomass.xe2x80x9d The culture of microorganisms is often termed xe2x80x9cactivated sludgexe2x80x9d and the mixture of the microorganism culture and the wastewater is often called xe2x80x9cmixed liquor.xe2x80x9d After the wastewater is in contact with the activated sludge for a period of time sufficient for treatment to have occurred, the mixed liquor is discharged to a secondary clarifier in which flocculation and settling of the sludge occurs to form a sludge blanket. During activated sludge treatment, the biomass in the system increases as particulate and soluble organic materials in the wastewater are converted into biomass by the microorganisms. Most of the settled activated sludge, often termed xe2x80x9creturn activated sludgexe2x80x9d or xe2x80x9cRAS,xe2x80x9d is returned to the aeration tank. A small portion, often termed xe2x80x9cwaste activated sludgexe2x80x9d or xe2x80x9cWAS,xe2x80x9d is removed from the process.
The performance of the activated sludge process can be significantly affected by the design of its unit operations (e.g., aeration and clarification) and by the manipulation of one or men more of three primary control variables: aeration rate, return activated sludge flow rate and waste activated sludge flow rate. Operation of the activated sludge process is optimized when those independent variables are correctly manipulated within a range of values. Since its inception to approximately the 1960s, the design of the activated sludge process was based primarily on experience, rules of thumb, process loading factors, etc. In the 1970s, the understanding of bacterial growth kinetics greatly advanced a more scientific approach to the design of activated sludge systems. With this knowledge it became apparentxe2x80x94and is now universally recognizedxe2x80x94that the growth rate of the microbial culture in the activated sludge process is the design and operational variable of most importance.
In essence, the microbial growth rate of activated sludge is the inverse of the mean cell residence time (known also as the sludge age, solids residence time, MCRT, or SRT). The MCRT is calculated by dividing the mass of microorganisms residing in an activated sludge system [typically taken as the mixed liquor total suspended solids (MLSS) or the mixed liquor volatile suspended solids (MLVSS)] by the mass of microorganisms removedxe2x80x94intentionally in the WAS stream and unintentionally in the secondary effluent steamxe2x80x94from the system per day.
Controlling the activated sludge process, like its design, has evolved since its inception. In the 1960s and 1970s, Alfred W. (Al) West of the U.S. Environmental Protection Agency (USEPA) developed control techniques that were initially based on sludge quality (West, A. W., Operational Control Procedures for the Activated Sludge Process, Part 1, Observations, Environmental Protection Agency Report No. EPA-330/9-74-001-a, April 1973). Three important elements of Al West""s techniques were a settlometer test, a centrifuge spin test, and a measurement of the secondary effluent turbidity. The settlometer test is conducted by collecting a sample of mixed liquor (i.e., the aeration basin contents) as it exits the aeration basin, shaking it, pouring it into a settling container, and recording the solids/liquid interface height (or depth) with respect to time, typically 0, 5, 10, 15, 20, 25, 30, 40, 50, and 60 minutes. The two most commonly used settling containers used are a I-liter (L) graduated cylinder and a 2-L Mallory settlometer. Although any container can be used (any see-through container, e.g., a canning jar), the 2-L Mallory settlometer is the most widely used today and is the container recommended by Al West. An important step in this procedure is plotting the interface height as a function of settling time and noting the general shape of the settling curve to ascertain the sludge""s settling characteristics.
The second key procedure to Al West""s techniques is the centrifuge spin test. In this test, an aliquot of the mixed liquor sample used in the settlometer test is centrifuged for 15 minutes and the percent volume of the initial aliquot volume occupied by the solids is recorded. He referred to this percentage as the xe2x80x9caeration tank concentrationxe2x80x9d or xe2x80x9cATC.xe2x80x9d The ATC was used as a surrogate for the mixed liquor suspended solids concentration, despite the fact that the ATC is a function of both the solids concentration and the structural integrity of the microbial cells and extracellular materials (e.g., the compressibility of the solids).
Finally, a sample of the secondary clarifier effluent is collected. The turbidity of this sample is measured because turbidity is much more quickly measured than the total suspended solids concentration.
There are at least two limitations of using Al West""s procedures to guide operation of the activated sludge process. First, while the compaction characteristics of a sludge are important, nowhere in an activated sludge system are the solids subjected to the multiple gravitational forces occurring in the centrifuge test Activated sludge solids are separated in almost all cases by gravity, so how the sludge compacts by simple settling is more appropriate. Second, the turbidity of the secondary clarifier effluent is confounded by the performance of the secondary clarifier(s). For this reason, it is inappropriate to use secondary clarifier effluent turbidity to quantify the flocculation characteristics of the activated sludge solids. Secondary effluent turbidity is, however, useful for quantifying the performance of the secondary clarifier when compared to the supernatant turbidity after the entering mixed liquor is flocculated and settled.
As the microbial growth kinetic approach to the design of activated sludge processes gained momentum, Al West expanded his approach to process control to take into account biomass growth kinetics. His approach is essentially one of controlling the SRT of the process.
Today, most approaches to activated sludge process control are based on growth kinetics. Essentially, the growth rate is controlled by one of three methods: (1) maintaining a constant SRT (or MCRT or sludge age), (2) maintaining a constant food-to-microorganism (F:M) ratio, or (3) maintaining a constant mixed liquor suspended solids or mixed liquor volatile suspended solids concentration or mass. For a variety of reasons (e.g., operator lack of understanding, influent variability, etc.), this approach is not working well.
The USEPA and others have documented the poor overall performance of the Nation""s activated sludge treatment plants. One EPA study (Environmental Protection Agency Report No. EPA-600/2-79-034) found that 50 to 90 percent of the plants investigated regularly violate treatment standards. That document also references earlier EPA studies that reported that a third to a half of wastewater treatment plants do not meet the standards for which they were built. Even though these studies were published 20 years ago, their findings are still relevant today for a very simple reason: activated sludge plants are becoming increasingly more difficult to operate due to the imposition of more stringent permit requirements, for example, nutrient removal. Clearly, the USEPA has documented the existence of a large unsolved problem. The problem is aggravated by the fact that quantification of unit process performance variability in activated sludge systems has been incompletely documented. Variability in effluent quality, on the other hand, has been studied. For example, many investigators have reported that effluent total suspended solids (TSS) and biochemical oxygen demand (BOD) concentrations follow a log normal distribution.
This disclosure shows that the problems associated with poor activated sludge process performance are primarily a function of two things: lack of activated sludge process understanding and over control of the activated sludge process. In attempting to control the growth rate of the activated sludge biomass by any of the common methods, an operator is, in essence, attempting to grow an activated sludge that settles, compacts, and flocculates well. The problem is that a test to measure a sludge""s settling, compacting, and flocculating characteristics is still not available. Operators"" focus on the growth rate is, therefore, one of convenience: because the SRT (or F:M or mixed liquor suspended solids concentration or mass) is measurable and the settling, compacting, and flocculating characteristics are not, operators focus on the former and xe2x80x9chopexe2x80x9d they get the latter. The fact remains, however, that sludge quality (i.e., its settling, compacting, and flocculating characteristics) is the parameter of utmost importance to activated sludge treatment plant performance.
The disclosures of a number of U.S. patents explain the activated sludge process and teach a variety of approaches to controlling the activated sludge process. The disclosures of the following U.S. patents is incorporated herein by reference as is fully set forth.
Mallory (U.S. Pat. No. 2,154,132) discloses a number of processes for controlling the activated sludge process. One process involves determining the suspended solids concentrations of both the return activated sludge and the aerated mixed liquor and manipulating the ratio of those concentrations to ensure that the ratio is substantially equal to the ratio between the aerator(s) volume and clarifier(s) volume, minus one (a value he calls xe2x80x9cthe plant constantxe2x80x9d). Another process involves determining the depth of the sludge blanket in the clarifier(s) and the amount of suspended solids in the aerator(s) and clarifier(s) and manipulating the ratio of those amounts to ensure that the ratio is substantially equal to the plant constant. A further process involves determining the ratio between the clarifier(s) volume and the volume of the sludge blanket and manipulating that ratio to be equal to the plant constant. Yet another process involves determining the ratio between the rate of effluent flow and the rate of return sludge flow and manipulating that ratio to be equal to twice the aerator(s) volume divided by the clarifier(s) volume, minus two. Another process involves determining the concentration of suspended solids in the aerated mixed liquor as it settled to form a sludge blanket and ensuring that the rate of increase in the concentration of suspended solids in the settling mixed liquor conforms to a specified straight line rate. In this process, the rate of increase in the concentration of suspended solids in the settling mixed liquor may be determined by measuring the rate at which the volume of the sludge blanket (as revealed by the location of the liquid/solids interface) in an unflocculated sample of mixed liquor decreases in a graduated cylinder. The reference also teaches the use of a centrifuge to measure suspended solids concentrations as an alternative to the conventional technique of measuring the dry weight of the solids.
The activated sludge process control techniques of the Mallory reference are limited in that many of the techniques require that multiple mixed liquor, waste activated sludge and/or return activated sludge suspended solids samples be taken and multiple concentration measurements to be made on those samples, measurements which are time-consuming, difficult to replicate and require special equipment. Other Mallory techniques require that measurements be made of the depth of the sludge blanket in the clarifiers, also a difficult measurement. None of the techniques involve measurement of the clarity of the supernatant above the sludge blanket in a sample of mixed liquor that has been previously flocculated as an indication of the flocculating character of the mixed liquor. Thus, the reference teaches away from a process control technique based on characterization of activated sludge settling, compacting and flocculating character.
Arthur (U.S. Pat. No. 3,746,167) discloses a method and apparatus for determining the amount of settleable and suspended solids in a liquid, said method and apparatus involving a filtration step. This reference is limited in that filtration of a wastewater sample through filters that can only be used once is required, rendering the method labor intensive. Moreover, the reference does not teach how to use the measurements to control the activated sludge process.
Chase et al. (U.S. Pat. No. 4,130,481) discloses a computer-implemented system for controlling the return rate of activated sludge flow in order to maintain the density of activated sludge in the aeration tank at an optimum value. Measured quantities include carbon dioxide respiration rate, aeration air flow rate, and the densities of mixed liquor and returned and stored activated sludge. This reference is limited in that a variety of complex analyzers are required, including a non-dispersive infrared analyzer. Moreover, the only process control technique used is varying the return activated sludge return rate, which ignores the importance of varying the waste activated sludge rate and the aeration rate to optimize the operation of the activated sludge process. The sludge volume index is mentioned, but only as a measure of the effectiveness of settling. No measurements of the character of activated sludge in the system are collected for use as inputs to control of the system.
Anderson (U.S. Pat. No. 4,168,233) discloses a system for automatically extracting samples of aeration tank mixed liquor and returned activated sludge, delivering a sample of mixed liquor to a settlometer jar for measurement of the sludge settling rate and delivering both a sample of mixed liquor and a sample of return activated sludge to a centrifuge for measurement of suspended solids concentrations. The solids/liquid interface in the settleometer is measured using a photo transistor sensing device (preferably, at 5 minute intervals for the first 30 minutes and at 10 minute intervals for the last 30 minutes), and the resulting settling curve is printed out. The reference suggests that the results of the testing can be used to develop control signals to vary the return activated sludge rate and/or the waste activated sludge rate (but not the aeration rate), but does not teach how to accomplish the critical step of using the collected data to affect activated sludge process control. The system of Anderson is limited in that no measurement is made of the height of the solids/liquid interface at set times in multiple, sequential samples and no measurement is made of the clarity (turbidity or suspended solids concentration) of settlometer supernatant (and, in fact, the supernatant is assumed to be xe2x80x9cclearxe2x80x9d). Thus, no measurement of the settling, compacting and flocculating character of the activated sludge is made. It is also limited in that a complicated apparatus (an automated centrifuge) must be used to measure sludge suspended solids concentrations.
Lang et al. (U.S. Pat. No. 4,170,553) disclose a process and apparatus for controlling the flocculation of foreign substances in a liquid that involves measurement of the turbidity of liquid drawn off from the process. The turbidity measurement is used to determine how much flocculating agent to add to the liquid for optimum treatment. This reference is limited in that it does not teach a system and method for control of the activated sludge process, but rather teaches control of a chemical flocculation step for water or wastewater.
Schreiber (U.S. Pat. No. 4,859,341) discloses a method for controlling an activated sludge process that involves measurement of the turbidity of clarified activated sludge obtained from the aeration tank and the concentration of dissolved oxygen in the aeration tank. The reference is limited in that it does not teach how a xe2x80x9cclarified waterxe2x80x9d is to be obtained from an xe2x80x9cactivated water-sludge mixture.xe2x80x9d The reference is further limited in that the settling, compacting and flocculating characteristics of the activated sludge are not measured.
Ballnus (U.S. Pat. No. 5,076,928) discloses a method for controlling the aeration of an activated sludge in a tank that involves measurement of the turbidity, biochemical oxygen demand or chemical oxygen demand of clarified liquid obtained from the tank. Reference is made in this patent to a prior U.S. patent by the same inventor (U.S. Pat. No. 4,333,838) which teaches using a xe2x80x9ccentrifuge or decanterxe2x80x9d to clarify liquid obtained from an aeration tank before the visible depth or turbidity of liquid is measured. These references are limited in that the settling, compacting and flocculating characteristics of the activated sludge are not measured.
Nader et al. (U.S. Pat. No. 5,173,187) discloses a method for controlling the biological clarifications stage of an activated sludge plant by monitoring the amount of certain microorganisms present using fluorescence-labeling and flow cytometry. This reference is limited by the use of complex measurement techniques to quantify the amount of particular xe2x80x9cproblemxe2x80x9d microorganisms that are present in an activated sludge.
LaPack et al. (U.S. Pat. No. 5,233,876) disclose an apparatus and methods for on-line analysis of the influent and effluent of gas, liquid and both gas and liquid process streams. This reference is limited in that the settling, compacting and flocculating characteristics of the activated sludge are not measured.
Ballnus (U.S. Pat. No. 5,242,592) discloses a method for controlling the aerator of an activated sludge tank that involves measurement of the turbidity in the wastewater and phosphate concentration in the mixed liquor suspended solids. This reference is limited in that the settling, compacting and flocculating characteristics of the activated sludge are not measured.
Watanabe et al. (U.S. Pat. No. 5,324,43 1) disclose a method for controlling the activated sludge process that involves monitoring the particle size of coagulating microorganisms and the length of filamentous microorganisms in the sludge. This reference is limited by the use of complex measurement techniques that focus on predicting the occurrence of increases in sludge volume index or sludge bulking. The reference is further limited in that the settling, compacting and flocculating characteristics of the activated sludge are not measured.
Norcross (U.S. Pat. No. 5,421,995) discloses a sludge blanket-clarified liquid interface detector. This reference is limited in that only measurement of the depth of the sludge blanket in a decanter and measurement of the turbidity of decanter effluent in a sequencing-batch-reactor process are taught. The reference is further limited in that the settling, compacting and flocculating characteristics of the activated sludge are not measured.
Neilsen (U.S. Pat. No. 5,589,068) discloses a method for automatically controlling a wastewater purification plant that comprises measuring at least two system parameters, deriving a control parameter on the basis of the measurements and reference to at least two control functions, selecting a control action on the basis of the control parameter and implementing it. The method preferably incorporates an evaluation of the quality of the measurements that relies on the use of a mathematical model of the plant as described in Denmark Patent Application No. 1677/91. The reference is limited in that the settling, compacting and flocculating characteristics of the activated sludge are not measured.
Salem et al. (U.S. Pat. No. 5,601,704) discloses an automatic feedback control system for a water or wastewater treatment apparatus that incorporates a recirculating solids contact clarifier. The system maintains steady-state operation of the clarifier based on the accurate measurements of the concentration of suspended solids at designated locations in the clarifier. The reference is limited in that the settling, compacting and flocculating characteristics of the activated sludge are not measured.
Okey et al. (U.S. Pat. No. 5,733,456) discloses control of a water/wastewater treatment system based on sensing the oxidation reduction potential at various locations in the system. The reference is limited in that the settling, compacting and flocculating characteristics of the activated sludge are not measured.
The disclosures of a number of related art publications explain the activated sludge process and teach a variety of approaches to controlling the activated sludge process. The teachings of each of these publications is summarized below.
Wahlberg, E. J., in Activated Sludge Bioflocculation: Measurement, Influencing Factors, and Physical Enhancement, Ph.D. dissertation, Clemson University, May 1992, pp. ii-iv and 31-33, 46-51, 85-87, 104-112, 122-124, disclosed a batch flocculation testing procedure for characterizing activated sludge flocculation characteristics. Mixed liquor was collected from the aeration tank and placed in 2-liter square jars. The contents of each jar were gently stirred by a paddle on a 6-paddle Phipps and Bird stirrer at a rotation velocity of 37.5 rotations per minute for a flocculation time that varied from 0 to 64 minutes. After 30 minutes of settling, supernatant was removed from each jar with a j-shaped tube connected to a vacuum pump, and its turbidity was measured. Aeration tank MLSS and MLVSS concentrations as well as the total SS concentration of the supernatant were also measured, and the data were used to determine flocculation equation parameters. This reference is limited in that a conventional stirrer is used in data collection. It is further limited in that the settling and compacting characteristics of the activated sludge are not measured.
Wahlberg, E. J. and Parker, D. S. in Troubleshooting Activated Sludge Secondary Clarifier Performance with Simple Diagnostic Tests, Florida Water Resources Conference, Tampa, Fla., Aug. 28-31, 1994, pp. 1-7, teach how to determine whether flocculation problems or clarifier short-circuiting problems are causing elevated activated sludge secondary clarifier effluent suspended solids concentrations. Measurement of dispersed suspended solids concentration is accomplished by performing a suspended solids analysis on supernatant siphoned off of a sample of mixed liquor after settling in a 4.2 liter acrylic Kemmerer sampler for 30 minutes after collection (without flocculation). Measurement of flocculated suspended solids concentrations is accomplished by performing a suspended solids analysis on supernatant of a sample of mixed liquor after 30 minutes of slow-speed stirring followed by 30 minutes of settling in an undisclosed apparatus. This reference is limited in that a Kemmerer sampler is used in data collection. It is further limited in that the settling and compacting characteristics of the activated sludge are not measured.
Wahlberg, E. J., Keinath, T. M. and Parker, D. S. in Influence of Activated Sludge Flocculation Time on Secondary Clarification, Water Environment Research, 66(6), September/October 1994, pp. 779-786, disclosed an apparatus and method for performing a flocculation test. The test was the same as that described in the Wahlberg (1992) reference described above. This reference is limited in that a conventional stirrer is used in data collection. It is further limited in that the settling and compacting characteristics of the activated sludge are not measured.
Wahlberg, E. J., Bower J., Bittner, M. and Margolis, Z. in Al West Meets W. Deming: A Statistical Approach to the Control of the Activated Sludge Process, WEFTEC""94, Water Environment Federation 67th Annual Conference and Exposition, Chicago, Ill., Oct. 15-19, 1994, teach how to apply statistical process control principles to activated sludge process performance monitoring. In the first phase of the study, secondary clarifier effluent turbidity measurements were used as the control variable. In the second phase, the following activated sludge characteristics were measured in a 2-liter Mallory settlometer once per day: zone settling velocity (i.e., the slope of a best fit line through the 0, 5, 10 and 15 minute interface height-versus-time data) was used to characterize the rate of settling, 30-minute settled sludge volume was used to characterize the extent of sludge compaction, and supernatant turbidity after 30 minutes of settling was used to characterize the degree of flocculation. In the third phase of the study, the slope of the line between the 0 and 5 minute interface heights was used to characterize zone settling velocity and measurements were performed on a set of five samples, with each set of samples collected at four times during each day. It is important to note that no initial flocculation step (gentle stirring) was performed prior to settling. A limitation of this reference is that conventional settlometers and conventional settling test protocols are used in data collection. Another limitation is that xe2x80x9cdegree of flocculationxe2x80x9d is incorrectly measured when the device and method disclosed in the reference are used as any flocs that exist in the aeration tank mixed liquor are broken up in the process of introducing the mixed liquor samples into the settlometers. This problem was unrecognized at this point in the development of the art. This reference thus teaches away from the previously published Wahlberg et al. references that teach how to perform other kinds of flocculation tests, which tests differ from the test disclosed herein. A further limitation is that no guidance is given as to how to use the collected data to implement specific control actions to optimize performance of the activated sludge process. The disclosure of this reference documents the status of the present inventor""s search for a better activated sludge process control method, but also shows that as late as 1994 that search was not over.
Water Environment Federation, Operation of Municipal Wastewater Treatment Plants, Manual of Practice No. 11, Fifth Edition, 1996, Water Environment Federation: Alexandria, Va., pp. 571-613, 646-675 and 688-689, describes activated sludge process variations and discloses standard practices for controlling the process. Methods are presented for aeration and dissolved oxygen control, return activated sludge control and waste activated sludge control. A xe2x80x9csludge qualityxe2x80x9d method for return activated sludge control involves development of a mixed liquor settleability curve and measurement of return activated sludge and aeration tank mixed liquor concentrations with a centrifuge. A xe2x80x9csludge qualityxe2x80x9d method for waste activated sludge control involves measurement of a variety of factors, including secondary effluent quality, mixed liquor dissolved oxygen concentration, oxygen uptake rate or respiration rate, mixed liquor, return activated sludge and waste activated sludge concentrations with a centrifuge and settled sludge concentration with a settlometer, typically at 5, 10, 15, 20, 25, 30, 40, 50 and 60 minutes with no flocculation. It is significant that this standard industry reference, which was authored by wastewater process control experts and published as late as 1996, teaches away from the devices and methods disclosed herein.
Wahlberg, E. J., Crowley, J. P., Bower, J., Bittner, M. and Margolis, Z. in Why the Activated Sludge Process Is So Hard to Operate: Modeling Brings New Light to Operations, WEFTEC""96, Water Environment Federation 69th Annual Conference and Exposition, Dallas, Tex., Oct. 5-9, 1996, disclose the conflicts that arise when using constant SRT, constant MLVSS and constant F:M ratio in waste activated sludge flow rate control strategies. A xe2x80x9cmore sludge quality based approach to controlxe2x80x9d of the activated sludge process is suggested but the reference is limited in that such an approach is not disclosed in it.
The above review of the background art reveals that no combination of references teach the invention disclosed herein. Even those references that suggest the use of xe2x80x9csludge qualityxe2x80x9d approaches to activated sludge process optimization do not teach the use of simple devices that allow for measurement of all of the important dimensions of sludge quality as required by this invention. No combination of references teach accurate measurement of the settling, compacting and flocculating characteristics of the activated sludge and/or use of such data for activated sludge process control. Even the references authored or coauthored by the inventor of the invention disclosed herein merely illustrate his up-until-now unsuccessful search for an elegant solution to the problem of activated sludge process optimization.
The purpose of the invention is to provide means for controlling and optimizing activated sludge process performance. The invention gives wastewater treatment plant operators a new tool for making the process-control decisions that affect activated sludge process performance. The invention provides an apparatus and a procedure for measuring a sludge""s settling, compacting, and flocculating characteristics. Moreover, the invention provides a technique for controlling the activated sludge process using the apparatus and procedure.
The invention is an apparatus and method for optimizing activated sludge process performance. The apparatus comprises means for measuring the following characteristics of an activated sludge: settling, compacting and flocculating. In a preferred embodiment, the apparatus comprises a motor platform, at least three (preferably four), vertically-oriented stator/legs connected to and extending below said motor platform, a motor attached to said motor platform, a vertically-oriented shaft extending below said motor, said shaft having a length that is less than the length of said statorAegs and having a mixer blade attached thereto. The apparatus is configured so that the stator/legs fit within a cylindrical container holding a sample of activated sludge mixed liquor with the bottoms of the stator/legs resting on the bottom of said container when the apparatus is in use. When flocculation is complete or when the apparatus is not in use, it is removed from the container and may be stored in the same orientation on the surface that the container is resting on by resting the bottoms of the stator/legs on the surface. The apparatus may also comprise the container, which container has an effective volume of about 2 liters. The container is fitted with a sampling port on its side through which a sample of the liquid near the top of the container may be removed. In a preferred embodiment, this sampling port is positioned at approximately one quarter of the height of the sample down from the water surface. In an alternative embodiment, a timer is used to control the length of time that each sample is stirred, settled and sampled.
In use, this embodiment is capable of mixing the sample and of being used to measure the settling character of the sludge, to measure the compacting character of the sludge, and to measure the flocculating character of the sludge. Mixing is accomplished using the motor to rotate the shaft at approximately 60 revolutions per minute (rpm) for approximately 30 minutes. While the appropriate rotational speed and duration can vary within the ranges 50 and 70 rpm and approximately 5-60 minutes for any particular sludge, it is important that essentially the same rotational speed and duration be used between tests. The settling character of the sludge is quantified by measuring the distance to which the interface between the solids and the liquids in the sample drops by approximately five minutes after stirring has stopped. Alternatively, the volume occupied by the settled solids approximately five minutes after stirring has stopped is used. The compacting character of the sludge is quantified by measuring the distance to which the interface between the solids and the liquid in the sample drops by approximately 30 minutes after stirring has stopped. Alternatively, the volume occupied by the settled solids approximately 30 minutes after stirring has stopped is measured and used. The flocculating characteristic is quantified by removing a sample of the supernatant liquid and measuring its turbidity or total suspended solids concentration (Water Environment Federation, Standard Methods for the Examination of Water and Wastewater, 20th Edition, 1998).
Another embodiment of the invention comprises the use of a Mallory settlometer fitted with stators and a stirring device to quantify the settling, compacting and flocculating characteristics of an activated sludge. The supernatant sample after stirring and settling can be obtained either from a sampling port tapped into the side of the settlometer or by withdrawing supernatant from the top of the settlometer using a siphon or other vacuum device. First, a two-liter (L) representative sample of activated sludge is removed from an aeration tank and placed in the modified settlometer. After the sample is flocculated, the interface between the liquid and the solids that comprise the sludge (termed the xe2x80x9cliquid/solids interfacexe2x80x9d) is considered to be at the top of the sludge sample.
The settling characteristic of the sludge is quantified by stirring the sludge for approximately 30 minutes at about 60 rpm, then ceasing the stirring and measuring the linear distance the solids/liquid interface has dropped (settled) in a given time, preferably approximately five minutes after the stirring has stopped. The linear distance can be divided by the settling time of five minutes to calculate the settling velocity (Vs) of the sludge. Alternatively, the volume occupied by the settled solids approximately five minutes after stirring has stopped is used.
The compacting characteristic of the sludge is quantified by measuring the degree to which the sludge has compacted in a given time, preferably approximately 30 minutes after the stirring has stopped. The linear distance that the solids/liquid interface has dropped after stirring has stopped is divided by the depth of the sludge sample (the original height of the solids/liquid interface measured from the bottom of the settlometer) to produce a proportion or percentage compaction. Alternatively, the volume occupied by the settled solids approximately 30 minutes after stirring has stopped is measured and used.
The flocculating characteristic of the sludge is quantified by measuring the turbidity of the liquid present at the top of the settlometer at a given time, preferably approximately 30 minutes after stirring has stopped. To make the measurement, a sample of liquid is removed from the settlometer 30 minutes after stirring has stopped and placed in a turbidimeter (e.g., a Nessler turbidimeter) for a determination of its turbidity (e.g., in Nephlometeric turbidity units or NTUS). Alternatively, this sample can be analyzed for its total suspended solids concentration (Water Environment Federation, Standard Methods for the Examination of Water and Wastewater, 20th Edition, 1998).
In yet another embodiment of the invention, the apparatus comprises means for removing a representative aliquot of activated sludge from an aeration tank or basin (preferably near the exit end of the tank or basin) and a reactor that provides means for mixing the sample, means for measuring the settling character of the sludge, means for measuring the compacting character of the sludge and means for measuring the flocculating character of the sludge. In one preferred embodiment, the aliquot of activated sludge is removed from the aeration tank by suction (i.e., it does not pass through a pump) to minimize uncontrolled disruption of its character. Suction may be provided by a pump located downstream of the reactor.
In this embodiment, mixing is provided by configuring the reactor so that eddy currents are caused when a shaft fitted with a mixing blade or paddle is rotated in the container. This may be accomplished by configuring the reactor so that it is non-circular (e.g., square) in horizontal cross section or by providing at least two stators that disrupt the circular motion of the aliquot when a motor-driven shaft having a mixing blade or paddle rotates in the reactor.
In this embodiment, means for measuring the settling character of the sludge is provided by fitting the apparatus with a means for measuring the distance by which the solids/liquid interface has dropped in a specified period of time after stirring has ceased, preferably in 5 or 10 minutes. If the walls of the reactor are transparent, optic means can be used, e.g., a light source on one side of the reactor and a vertically-oriented series of photocells on the other side to determine the height of the solids/liquid interface. Alternatively, a turbidity sensor is lowered into the reactor until a large increase in turbidity is sensed, indicating the level of the solids/liquid interface or an ultrasonic probe is used to sense the level of the solids/liquid interface.
In this embodiment, means for measuring the compacting character of the sludge is provided by fitting the apparatus with a means for measuring the distance by which the solids/liquid interface has dropped in a second specified period of time after stirring has ceased, preferably in 30 minutes. In a preferred embodiment, the same means is used to measure this distance as is used to measure the settling character of the sludge.
In this embodiment, means for measuring the flocculating character of the sludge is provided by fitting the apparatus with means to measure the turbidity of the liquid that has separated from the sludge during settling. In a preferred embodiment, this is accomplished by slowly (so as not to disturb the settled solids) removing a sample of the liquid from the top of the reactor and measuring its turbidity in a turbidimeter.
Still another embodiment of the invention comprises collection of a first group of data on the settling, compacting and flocculating characteristics of an activated sludge at a first point in time and then collection of a second group of data on the settling, compacting and flocculating characteristics of an activated sludge at a second, subsequent point in time. The first group of data are compared to the second group of data to determine whether the settling, compacting and/or flocculating characteristic(s) of the sludge has changed significantly, and, if so, in which direction (e.g., increase or decrease). A knowledgebase of rules (preferably in the form, IF  less than predicate greater than THEN less than consequent greater than ) concerning how to modify the aeration rate, return activated sludge flow rate and the waste activated sludge flow rate depending on whether sludge settling, compacting and flocculating characteristics have changed and, if so, in which direction, is then accessed. An inference is made as to the activated sludge process operating strategy that is most likely to optimize the performance of the process by processing said rules. The operational strategy is then implemented to control the activated sludge process and produce the best quality effluent.
In a further embodiment, the invention is an activated sludge treatment plant controlled using the process disclosed herein. In a preferred embodiment, the activated sludge treatment plant includes an influent pumping station, a headworks, primary sedimentation tank(s), activated sludge aeration basin(s), secondary clarifier(s), flow-adjustable RAS pump(s), flow-adjustable WAS pump(s), flow-adjustable blower(s), disinfection facilities, sludge stabilization units and process control features.
In broad terms, a preferred embodiment of the apparatus is comprised of means for measuring, at a plurality of points in time, the settling character of an activated sludge, the compacting character of an activated sludge and the flocculating character of an activated sludge. Other embodiments of the invention further comprise means for processing the measurements so produced by, for example, statistical process control techniques, to produce appropriate operational strategies for activated sludge process control and/or troubleshooting. Yet other embodiments of the invention further comprise a wastewater treatment plant being operated as disclosed herein.
In broad terms, a preferred embodiment of the method is comprised of the following steps: measuring, at a plurality of points in time, the settling character of an activated sludge, the compacting character of an activated sludge and the flocculating character of an activated sludge and processing the measurements so produced using a knowledgebase or rule set to produce appropriate operational strategies for activated sludge process control. Other embodiments of the invention involve a rule set for selection of optimum activated sludge process control strategies. Other embodiments involve computer control of an activated sludge process based on said strategies. Other embodiments involve troubleshooting of an activated sludge process based on said strategies.
The present invention calls for quantification of the inherent variability in the activated sludge process before taking control actions. The most likely result of not quantifying this variability is xe2x80x9cover control;xe2x80x9d that is, making a process control change when no change is warranted. This often occurs when a change in the quality characteristic (i.e., a measurable performance variable) occurs, but that change is completely within the normal variability of the system.
The present invention is based on the recognition that it is inappropriate to measure the quality of the end product without measuring the quality of the intervening steps taken to make that product (e.g., sludge quality) and expect to consistently meet performance specifications. It recognizes that it is inappropriate, therefore, for an operator of an activated sludge plant to base control decisions on final effluent quality as is often done today. This disclosure illustrates that, in fact, controlling the quality of the activated sludgexe2x80x94ensuring a sludge is grown that settles, compacts, and flocculates wellxe2x80x94is absolutely key to successful activated sludge operation. Unfortunately, an easily-performed but meaningful method of sludge quality measurement does not exist. The invention described herein is designed to address this void. In addition, the invention provides that the tests disclosed herein be conducted in such a way that the data provide a means for quantifying system (along with sample and analytical error, although these tests attempt to minimize these errors) variability. It is with an understanding of this variability that actual changes in the process can be seen and responded to.
One advantage of the invention is that it provides a simple means for measuring the settling character of an activated sludge, the compacting character of an activated sludge and the flocculating character of an activated sludge, these characteristics being tantamount to the successful performance of the activated sludge process. These characteristics of an activated sludge can have a huge impact on how an activated sludge system will perform, how large a plant must be built, and/or what the capacity of an existing plant is. Yet, before this invention, no one had a way of measuring all of these very important sludge quality characteristics. Another advantage of the invention is that it provides a rational approach to activated sludge plant operation. Yet another advantage of the invention is that it can be used by wastewater treatment plant operators to determine whether a change should be made in the operation of an activated sludge process, and, if a change is appropriate, the nature and extent of the change. A further advantage of the invention is that it provides a wastewater treatment plant that operates as it was intended to operate during its design and when expansion of that design is necessary. Another advantage of the invention is that it allows measurement of the flocculating character of an activated sludge to be performed as part of a test that most operators already do: the settlometer test. A further advantage of the invention is that it incorporates the use of statistical process control to quantify the variability of control parameters in a way that prevents over control of the activated sludge process.
The art of activated sludge process control has not changed for decades. The short schools, correspondence courses, associate degrees and certification exams teach the same approach: kinetic control. The present invention represents a paradigm shift in at least two significant ways: focusing on sludge quality characteristics and quantifying their variability. With this approach, a goal that has eluded operators for decades can be achieved: activated sludge process optimization.
Further aspects of the invention will become apparent from consideration of the drawings and the ensuing description of preferred embodiments of the invention. A person skilled in the art will realize that other embodiments of the invention are possible and that the details of the invention can be modified in a number of respects, all without departing from the inventive concept. Thus, the following drawings and description are to be regarded as illustrative in nature and not restrictive.